1. Field of the Invention
The present invention relates to a compression ignition internal combustion engine.
2. Description of Related Art
In a compression ignition internal combustion engine, fuel injected from an injector in the form of liquid droplets is atomized and vaporized, and is mixed with air to form a combustible mixture gas. Then, the combustible mixture gas is ignited. Thus, the fuel is combusted in the compression ignition internal combustion engine. The injected fuel reaches a combustible state after the fuel is atomized and the combustible mixture gas is formed. Therefore, ignition timing lags behind fuel injection timing. More specifically, an ignition delay exists between the fuel injection timing and the ignition timing. Generally, in the combustion, premixed combustion occurs first, and then, diffusion combustion, of which speed is limited by mixing speed of the fuel and the air, occurs. In an early stage of the combustion (or in the premixed combustion), the fuel and the air are mixed sufficiently uniformly and the combustion occurs rapidly. Therefore, a heat release rate pattern in the combustion process has a peak in the early stage of the combustion, at which the heat release rate rises largely.
In recent years, reduction of exhaust emission and noise of the internal combustion engine has been emphasized more and more, in addition to improvement of motive performance. Exhaust gas recirculation (EGR) for recirculating exhaust gas into an intake system is well known as one of the technologies for reducing the exhaust emission. The premixed combustion causes the rapid combustion and increases temperature inside a cylinder. Therefore, the EGR is performed to reduce oxygen concentration so that an excessive increase of temperature inside the cylinder is prevented. Thus, nitrogen oxides can be reduced.
If the EGR is performed, the nitrogen oxides can be reduced but the decrease of the oxygen concentration is inevitable. Accordingly, smoke can be easily generated in the diffusion combustion, in which the fuel and the air are not mixed sufficiently. Therefore, a technology disclosed in Japanese Patent Publication No. 2864896 (Patent Document 1) retards the fuel injection timing till after a top dead center (TDC) to extend the ignition delay when the EGR is performed so that the generation of the smoke is inhibited. This technology extends the ignition delay extraordinarily so that the entity of the injected fuel is brought to the premixed state before the ignition.
In a technology disclosed in Unexamined Japanese Patent Application Publication No. 2001-165017 (Patent Document 2), multiple injection holes are formed and a passage area of each injection hole is reduced. Thus, the mixed state of the fuel and the air can be improved efficiently.
However, in the technology disclosed in Patent Document 1, the ignition is started after the entity of the injected fuel reaches the premixed state. Therefore, a large amount of the fuel is combusted at once. Accordingly, the heat release rate rises at once, and then, decreases rapidly. A heat release rate pattern provides a peak-shaped profile as a whole. Therefore, the noise increases. The fuel injected in the early stage of the injection diffuses during the long ignition delay. Accordingly, there is a possibility that the mixed gas becomes excessively lean and hydrocarbon or carbon monoxide is generated. Since the injection timing is retarded and the ignition delay is extended extraordinarily, the influence on the efficiency is inevitable, and fuel consumption is deteriorated.
The technology disclosed in Patent Document 2 promotes the atomization of the fuel and can shorten the ignition delay. However, even if the technology of Patent Document 2 is applied to the technology of Patent Document 1, the overall peak-shaped profile of the heat release rate pattern cannot be changed. As a result, the reduction of the nitrogen oxides or the smoke cannot be achieved at the same time as the improvement of the noise prevention ability and the fuel consumption.